An ultraviolet-hardening resin composition is composed of an acrylic oligomer, an acrylic monomer and a photo-polymerization initiator as essential components. The oligomer is a main factor of determining the physical properties of the hardened product from the composition and the liquid properties of the composition itself. As typical examples of the oligomer, there are mentioned urethane acrylates, epoxy acrylates and polyester acrylates.
In particular, urethane acrylates, which can be characterized in flexibility, toughness, weather resistance, chemical resistance and glass transition point by selecting the kinds and proportions of the components such as polyol component, polyisocyanate component and hydroxyl group-containing acrylic monomer, are used in various fields of woodworking materials, polyvinyl chloride flooring materials, coating agents for shaped plastic articles, and coating agents for optical fibers.
Coating agents themselves, containing such urethane acrylates, are described in JP-B 54-22238 and 57-34853. (The term "JP-B" as used herein means an "examined Japanese patent publication".) However, the hydroxyl group-containing acrylates as disclosed in them are only 2-hydroxyethyl acrylate and 2-hydroxypropyl acrylate, and only 2-hydroxyethyl acrylate is concretely used in the examples of them. The disclosed acrylates could not be incorporated into the coating agent in the absence of a solvent.
On the other hand, JP-A 1-161010 mentions a coating agent containing an urethane acrylate. (The term "JP-A" as used herein means an "unexamined published Japanese patent application".) However, it relates to an electron-hardening resin composition, which is different from the present invention relating to an ultraviolet-hardening resin. In addition, only 2-hydroxyethyl acrylate is used in the examples of JP-A 1-161010, and this indispensably needs an acrylic monomer as a reactive diluting agent.
JP-A 1-185313 relates to a photo-hardening resin, like the present invention, and the use of urethane acrylates is mentioned. However, only 2-hydroxyethyl acrylate is used in the examples of JP-A 1-185313, and it also needs a reactive diluting agent of an acrylic monomer or the like.
The conventional urethane acrylate oligomers as disclosed in the above-mentioned patent publications or applications have a high viscosity of themselves, as having many urethane bonds therein, and therefore need various diluting agents (such as acrylic monomers, solvents, etc.) when they are formed into coating agents or coating paints. Thus, they involve various problems with respect to the current needs of solvent-free products and energy saving products.
In addition, they are also not sufficiently satisfactory with respect to the ultraviolet-hardening speed and to the physical properties of the hardened films from them, such as flexibility, heat resistance and weather resistance of hot-cold repetition cycles.